1. Field of the Invention
The present invention relates to a fuel cell stack, and more particularly to an oxidizer flow path of a fuel cell stack.
2. Description of the Related Art
A polymer electrolyte fuel cell basically includes a polymer electrolyte membrane having proton conductivity, and a pair of catalytic layers and electrodes arranged at both sides of the polymer electrolyte membrane.
The catalytic layer is generally composed of platinum or a platinum-group metal catalyst. A gas diffusion layer that supplies gas and collects electricity is provided at the outer surface of the catalytic layer.
An assembly in which the polymer electrolyte membrane and the catalytic layer are integrated is referred to as a membrane electrode assembly (MEA). In the membrane electrode assembly, fuel (hydrogen) is supplied to one electrode, while oxidizer (oxygen) is supplied to the other electrode, whereby electricity is generated during the process of generating water.
The electrode to which the fuel is supplied is referred to as a fuel electrode, while the electrode to which the oxidizer is supplied is referred to as an oxidizer electrode. Power is taken out from the electrodes at both sides.
The theoretical voltage of a fuel cell unit made of a membrane electrode assembly is approximately 1.23 V. In a normal operation state, the fuel cell unit is mostly used with the theoretical voltage being set to about 0.7 V.
Therefore, in a case where it requires a higher activation voltage, a plurality of fuel cell units are stacked and arranged electrically in series to be used.
The stack structure described above is referred to as a fuel cell stack. Usually, the oxidizer flow path and the fuel flow path are isolated from each other by means of a member, which is called a separator, in the stack. A recess/protrusion pattern (groove) is formed in the respective plate-shaped separators, wherein the recessed portion facing the membrane electrode assembly is configured as a gas flow path, while the protruding portion is configured as a current-collecting portion.
In a fuel cell used in a portable electronic device, air that is the oxidizer is taken in such that outside air is directly supplied due to a natural diffusion or a ventilation means such as a fan. In the stack structure, air is taken in from only a side surface of the stack.
In the fuel cell stack described above, a plurality of cell units simultaneously generate electricity. However, since the plurality of cell units are stacked, the rate of heat radiation differs for each part.
Specifically, heat is more likely to be accumulated in the cell units located at the central position in the stacking direction, while heat is more likely to be radiated at the cell units located at both ends.
Therefore, a temperature distribution will be formed such that the temperature is the highest at the central part and the temperature is relatively lower at both ends in the stacking direction of the fuel cell stack.
Due to the temperature distribution, each cell unit of the fuel cell stack will generate electricity under the different temperature condition.
Accordingly, the disadvantages described below likely to occur.
Firstly, a so-called flooding phenomenon is more likely to occur at the cell units located at the uppermost part or the lowermost part in the stacking direction.
The flooding phenomenon refers to a phenomenon in which water generated at the oxidizer electrode is condensed, and the condensed water degrades the gas diffusion property in the oxidizer electrode to thereby cause degradation of the characteristics. When the temperature distribution occurs in the stack, water is easier to be condensed at the cell unit having a low temperature. Therefore, the flooding phenomenon is more likely to occur at the cell units located at both ends.
A so-called dry out phenomenon is likely to occur at the cell units located at the central part in the stacking direction. The dry out phenomenon refers to a phenomenon in which water content in the polymer electrolyte decreases with the temperature rise, which increases the internal resistance in the cell unit to thereby cause degradation of the characteristics.
Since water, which is generated at the oxidizer electrode, is more rapidly evaporated at the cell unit having a higher temperature, the dry out phenomenon is more likely to occur at the cell units located at the central part.
In order to eliminate the instability of the characteristics due to such temperature distribution, Japanese Patent Application Laid-Open No. 2005-340173 proposes a fuel cell stack in which a distribution is given to the air-supply amount to each cell unit of a fuel cell stack.
In this fuel cell stack, the sectional area of an oxidizer flow path formed in a separator of each cell unit, which have the low temperature and are located at both ends, is set to be the largest.
By virtue of this structure, the amount of taking in the supplied air becomes large at the both ends. Therefore, even when the temperature is lower, water is hardly condensed, so that the variation in the degree of occurrence of the flooding phenomenon in the stacking direction is reduced.
Furthermore, Japanese Patent Application Laid-Open No. 2004-311279 proposes a fuel cell in which the sectional area of an oxidizer flow path of each cell unit at the central part in the fuel cell stack is set to be the largest.
This fuel cell is configured such that the air-supply amount is set to be the largest at the central part so as to increase the radiation amount of the cell units at the central part, due to heat exhaust through the air, thereby suppressing the dry out phenomenon.
However, the above-mentioned conventional fuel cells have the problems described below.
Specifically, in the fuel cell stack disclosed in Japanese Patent Application Laid-Open No. 2005-340173, the power generation temperature of the stack may vary greatly depending on the output required for the device or the operation environment factors.
Furthermore, the temperature distribution tends to increase, as the temperature of the whole stack becomes high. Therefore, when the temperature of the whole stack is low, the chances are small that the temperature of the cell units at the central part is prominently high.
In the fuel cell disclosed in Japanese Patent Application Laid-Open No. 2005-340173, the sectional area of the oxidizer flow path of the cell units at the central part in the stacking direction is relatively smaller, so that the amount of taking in air is reduced.
Therefore, when the power generation temperature of the fuel cell stack is not sufficiently raised, the flooding phenomenon is likely to occur at the cell units at the central part instead. However, this patent document does not disclose any countermeasure against such situation.
Apart from the external humidification type fuel cell stack in which humidified fuel or oxidizer is supplied, the technology disclosed in Japanese Patent Application Laid-Open No. 2004-311279 is not necessarily effective in a self-humidification type fuel cell stack that does not humidify the supplied gas.
Since the self-humidification type fuel cell stack directly takes in air from the outside, the humidity of the supplied air becomes relatively small, if the power generation temperature of the stack is higher than the outside temperature.
Supplying the air having such low humidity in a large amount to the central part that is likely to have a high temperature may allow the dry out phenomenon to be easy to occur.